fluxgate-based displacement sensor

TIDA-00463

DRV425 DRV425

VOUT1

VOUT2REFOUT1

ADS1248

MISO

MOSI

CS

CLK

3.3 V

TPS71745ENABLE

4.5 V

ERROR1OR1 OR2 ERROR2

3.3 V3.3 V

VUSB

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1TIDUB46–December 2015Submit Documentation Feedback

Copyright © 2015, Texas Instruments Incorporated

Fluxgate-Based Displacement Sensor

TI DesignsFluxgate-Based Displacement Sensor

All trademarks are the property of their respective owners.

TI DesignsThe TIDA-00463 showcases a displacement sensor forlinear distance measurements in a BoosterPack™form factor for TI LaunchPad™.The design is suitable for any application where adistance measurement, even through non-magneticmaterials, is needed and non-contact, wear-freemeasurements are required.Through the use of magnetic fluxgate technology, thisdesign is able to cover a wide measurement distancewhile achieving high resolution and accuracy.

Design Resources

TIDA-00463 Tool Folder Containing Design FilesDRV425 Product FolderADS1248 Product FolderTPS71745 Product Folder

ASK Our E2E Experts

Design Features• Measures 10-cm Distance, 1-mm Resolution With a

6-mm diameter NdFeB Magnet (HC > 1350 kA/m)• Detects the Presence of the Magnet up to 30 cm• Common Mode Fields Rejection• Temperature Range: –40°C to 125°C• No Temperature Dependence:

Max Temp Drift = 100 nT/°C• Non-Contact, Wear-Free Measurements• Detects Even Through Non-Magnetic Materials• BoosterPack Form Factor, Compatible With TI

LaunchPad

Featured Applications• Factory Automation and Process Control• Sensors And Field Transmitters• Robotics

An IMPORTANT NOTICE at the end of this TI reference design addresses authorized use, intellectual property matters and otherimportant disclaimers and information.

http://www.go-dsp.com/forms/techdoc/doc_feedback.htm?litnum=TIDUB46

http://www.ti.com/tool/TIDA-00463

http://www.ti.com/product/DRV425

http://www.ti.com/product/ADS1248

http://www.ti.com/product/TPS71745

http://e2e.ti.com

http://e2e.ti.com/support/applications/ti_designs/

Key System Specifications www.ti.com

2 TIDUB46–December 2015Submit Documentation Feedback



1 Key System SpecificationsThe following parameters depend on the magnet used in the application. For the TIDA-00463, the chosenmagnet (NdFeB) has a diameter of 6 mm and a HC > 1350 kA/m.

Table 1. System Parameters

PARAMETER SPECIFICATION DETAILSMax detectable distance 30 cm See Section 7.1Resolution 1 mm (from 2 to 10 cm) See Section 7.1Response time Dependent from ADC data rate See Section 3.6.2CM fields rejection See Section 3.5Noise (peak-to-peak) 320 nT See Section 7Temperature –40°C to 125°C See Section 3.6.3Max temp drift 100 nT/°C See Section 7.3Supply voltage 4.5 V See Section 3.6.3

2 System DescriptionThis design is meant for contactless magnetic-based linear distance measurement using fluxgatetechnology integrated in the DRV425.

The TIDA-00463 eliminates big common mode field interferences (for example, the earth magnetic field)thanks to the adopted differential approach where the outputs of the two sensors are measureddifferentially. Nevertheless, it is possible to measure the output of each sensor independently being ableto estimate as well, if present, any common mode field.

The solution is available in a BoosterPack form factor and compatible with TI LaunchPad to easily use andevaluate.

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www.ti.com System Design Theory




3 System Design TheoryThe system is designed for linear distance measurements where the distance is dependent on themagnetic field generated by any magnet. The measured distance depends on the strength and dimensionof the magnet, stronger is the magnet bigger is the distance that the sensor is able to measure.

The used sensor is the DRV425, which provides the unique and proprietary, integrated fluxgate sensor(IFG) with an internal compensation coil to support a high-accuracy sensing range of ±2 mT with ameasurement of up to 47 kHz.

3.1 Magnetic SensorsMagnetic field strength is measured using a variety of technologies. Each technique has unique propertiesthat make it more suitable for particular applications. A magnetic field is one of the most important physicalquantities that have been used in many applications such as: research of magnetic materials, geophysics,space, navigation system (detecting wafting goods), mapping of earth's magnetic field, electroniccompass, determination of object's position or sensor apart in small order, and many other applications [1][2]. These applications can range from simple sensing to precise measurements of a magnetic field.Based on magnetic field strengths and measurement range, magnetic field sensors can be divided intotwo categories:1. Sensors that measure low fields (< 1 mT), commonly called magnetometers2. Sensors that measure high fields (> 1 mT), commonly called Gauss meters [3]

Magnetometers can be divided into vector component and scalar magnitude types. Vector magnetometersmeasure the vector components of a magnetic field while scalar magnetometers measure the magnitudeof the vector magnetic field.

3.2 Fluxgate TechnologyThe heart of a fluxgate magnetometer is the fluxgate. It is the transducer that converts magnetic field intoelectric voltage. The fluxgate is the most widely used magnetic field vector measuring instrument. It isrugged, reliable, and relatively less expensive than the other low-field vector measuring instruments.These characteristics, along with its ability to measure the vector components of magnetic fields over a0.1-nT to 1-mT range from DC to several kHz, make it a very versatile instrument.

Geologists use them to explore, and geophysicists use them to study the geomagnetic field (about20 to 75 μT on the Earth's surface). Satellite engineers use them to determine and control the attitude ofspacecraft, scientists use them in their research, and the military uses them in many applications,including mine detection, vehicle detection, and target recognition. Some airport security systems usethem to detect weapons [2] [3].

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System Design Theory www.ti.com




3.2.1 Electromagnet PrincipleThe fluxgate main principle uses a conductor, which carries the current of interest. The current generatesa magnetic field; through this magnetic field measured by the device, it is possible to calculate the currentflowing in the conductor.

Using Faraday's law of induction, where the induced EMF ~ dB/dt, estimate the changing (AC) currentusing a secondary winding (transformer); to measure the constant field, the user needs a magneticsensor.

(1)

Figure 1. Ampere’s Law (Also Applies to DC Currents)

3.2.2 General Mode of OperationThe fluxgate magnetometer consists of a ferromagnetic material core with two coils, an excitation, and asense coil (see Figure 2). It exploits magnetic induction together with the fact that all ferromagneticmaterial becomes saturated at high fields. This saturation can be seen in the hysteresis loops shown onthe right side of Figure 2 [4].

Figure 2. Illustration of Operating Principles of Fluxgate Magnetometers

NOTE: The output signal becomes modulated by driving the soft magnetic core into and out ofsaturation. The shaded regions indicate the regions of operation.

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The core is magnetically saturated alternatively in opposing directions along any suitable axis by means ofthe excitation coil driven by a sine or square waveform. Prior to saturation, the ambient field is channeledthrough the core producing a high flux due to its high permeability. At the point of saturation, the corepermeability falls away to the vacuum, causing the flux to collapse. During the next half cycle of theexcitation waveform, the core recovers from saturation, and the flux due to the ambient field is once againat a high level until the core saturates in the opposite direction; the cycle then repeats [5].

The voltage output from the sense coil consists of even-numbered harmonics of the excitation frequency.For readout, the second harmonic is extracted and rectified. The voltage associated with this harmonic isproportional to the external magnetic field [4].

Figure 3. Ideal Fluxgate Waveforms

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r

2 2 22

B h z zB z

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3.2.3 SaturationSaturation is a state reached when an increase in applied magnetizing field H cannot increase themagnetization of the material further, so the total magnetic field B levels off. As the H field increases, theB field approaches a maximum value asymptotically, the saturation level for the substance. The magneticfield represents the existing current trough a conductor. From this idea, the excited saturable inductor isable to measure current.

The saturation point of ferromagnetic materials depends on magnetic permeability and also depends onthe amount of current. The core permeability will change both by an external field (produced by externalcurrent IPRIM) and an excitation current (IEXCT) through the inductor [6].

Figure 4. Magnetization Curve and B-H Curve

3.3 Specifics for Disc MagnetsThe static magnetic field of a disc magnet is dependent on its magnetic remanence Br, height h, andradius r. The magnetic flux density at a certain distance z from the surface of the magnet is calculated to

(2)

where• Br is the magnetic remanence• h is the height• r the radius of the disc magnet

The magnetic field strength is equal for both distances in either direction of the disc magnet. Only themagnetic field lines are orientated oppositely. Depending on the orientation of the magnet, compared tothe electric field inside the conductor, the measured voltage is either of positive or negative sign.

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3.4 Simulating Magnetic Field for System DesignThough many tools can be used to simulate magnetic field, this section details using the tool called FEMM4.2 [6]. This tool uses the finite element method to calculate magnetic fields. The finite element method isa numerical method to calculate solutions for partial differential equations. In principle, the area in whichthe magnetic field is propagating is divided into several small elements with finite size. For every element,a basic function is defined. Together with boundary and transition conditions, these basic functions areinserted into the general partial differential equation. Then, the resulting system of equations is solvednumerically. A detailed equation about the finite element method can be found in The Finite ElementMethod [7]. To explain the handling of the tool, see Figure 5. FEMM 4.2 calculates the magnetic field foraxis symmetric objects, the y axis serves as rotation axis. A complete documentation of the tool can befound online at www.femm.info.

Follow these tips on how to run a FEMM 4.2 simulation if the user is not familiar with it:• Define the dimensions of the magnet and its surrounding area. The box in the middle on the enlarged

part of Figure 5 is the magnet, the outer area is air. The magnet has a diameter of 6 mm and a heightof 5 mm.

• Set the properties of the defined areas. In this case, the orientation of the magnetization direction is setto 90° and the coercive field strength HC is set to 1350 kA/m.

• For the surrounding area, specify as property the air. It is best practice to define a rectangular area(property: air) in the same direction the user wants to calculate the magnetic field.

• Now, simulate the magnetic field.

Figure 5. FEMM Simulation

The result is shown in Figure 5. The thin lines indicate the magnetic flux lines. The higher the density ofthe magnetic flux lines, the higher the magnetic flux density. In addition, this relation is also pointed out bydifferent colors. Magenta indicates a high magnetic flux density, and cyan indicates a low magnetic fluxdensity.

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http://www.femm.info

Distance (mm)

B (

mT

)

0 200 400 600 800 10000

0.2

0.4

0.6

0.8

1

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2.2

2.4

D001Distance (mm)

B (

µT

)

300 400 500 600 700 800 900 10000

5

10

15

20

25

30

35

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45

50

D002





Furthermore, the tool can display the strength of the magnetic flux density along a user defined line.Therefore, a 1-m long line, from left to right perpendicular to the cross section of the magnet is drawn. Theresulting curve is shown in Figure 6. The picture on the right side is an enlarged version of the one on theleft.

Considering that the sensitivity of the DRV425 is 12.2 mA/mT with a shunt resistance of 20 Ω and abandwidth setting equal to 0, the bandwidth of the sensor is equal to 30.738 kHz, meaning a noise densityof 263 nT. This means that our sensor with this magnet, in absence of other magnetic sources, canmeasure up to a 1-m distance.

Figure 6. Magnetic Flux Density versus Distance

3.5 Differential MeasurementsUnder certain conditions, the DRV425 can measure up to hundreds of nT. However, considering that theearth magnetic field is in the range between 25 and 65 μT, the high sensitivity of the sensor becomesuseless if the user does not calibrate before using.

In addition, the user should calibrate the system periodically because the earth magnetic field varies withtime and with respect to the position of the system. Calibration requires additional time and resources.

To avoid calibration, the TIDA-00463 uses two DRV425 and measures the difference of the two outputs.The drawback of this solution is that the distance range is smaller with respect to the solution where onlyone sensor is used because information is given by the difference of the two voltage outputs: the closerthe sensors are, the smaller the difference of the two voltage outputs and the measured distance become.

On the contrary, if the two sensors are far from each other, the common mode field influences detected bythe two could not be anymore the same. Therefore, they will not be completely cancelled.

For the TIDA-00463, it was found that a good compromise in terms of distance between the two sensorsis 10 mm. For differential measurements, the direction of the two sensors should be the same so that thecommon mode fields will cancel out.

Finally, the TIDA-00463 can measure the single output of the sensors to monitor the common mode fields.

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3.6 Analog Signal Chain

3.6.1 Magnetic Field SensingThe DRV425 is chosen because of its high sensitivity to any magnetic field that can measure biggerdistances with respect to any other magnetic technology.

In addition the low offset, offset drift, and noise of the sensor, combined with the precise gain, low gaindrift, and very low nonlinearity provided by the internal compensation coil, result in unrivaled magnetic fieldmeasurement precision.

Furthermore, it offers a complete set of features, including an internal difference amplifier, on-chipprecision reference, and diagnostic functions to minimize component count and system-level cost.

The following procedure was used to design a solution for a linear-position sensor:• Select the proper supply voltage (VDD) to support the desired magnetic field range (see Table 2).• Select the proper reference voltage (VREFIN) to support the desired magnetic field range and to match

the input voltage specifications of the desired ADC.• Use the DRV425 System Parameter Calculator, (SLOC331; RangeCalculator tab) to select the proper

shunt resistor value of RSHUNT.• The sensitivity drift performance of a DRV425-based linear position sensor is dominated by the

temperature coefficient of the external shunt resistor. Select a low-drift shunt resistor for best sensorperformance.

• Use the DRV425 System Parameter Calculator (Problems Detected Table in DRV425 SystemParameters tab) to verify the system response.

The TIDA-00463 allows the user to read back the error and over-range flag of the two sensors, readingthem back through the LaunchPad.

Table 2. Design Parameters

DESIGN PARAMETER EXAMPLE VALUE

Magnetic field range VDD = 5 V: ±2 mT (max)VDD = 3.3 V: ±1.3 mT (max)

Supply voltage. VDD 3.0 to 5.5 V

Reference voltage, VREFINRange: GND to VDDIf an internal reference is used: 2.5 V, 1.65 V, or VDD / 2

Shunt resistor, RSHUNTDepends on the desired magnetic field range, reference, and supply voltage; see theDRV425 Systems Parameter Calculator (SLOC331) for details.

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http://www.ti.com/lit/pdf/SLOC331

http://www.ti.com/lit/pdf/SLOC331

OUT

mVV B 976

mT= ´

OUT SHUNT AMPV B G R G= ´ ´ ´





Figure 7 shows the parameters selected for the TIDA-00463.

Figure 7. TIDA00-463 System Parameters

The maximum measurable magnetic field is 1.9 mT and system bandwidth is around 30 kHz. The tooldetects only a warning over temperature related to the over-range flag, which it might not trigger.However, this is only a warning and it disappears if the max temperature is equal to 85°C. The output ofthe sensor is directly proportional to the measured magnetic field and it is calculated using Equation 3:

(3)

where:• B is the measured magnetic field• G = 4 is the gain of the integrated differential amplifier• RSHUNT = 20 Ω is the external shunt resistor• G = 12.2 mA/mT is the sensor gain

Substituting these values to Equation 3:

Considering that the maximum measurable magnetic field is 1.9 mT, the maximum measurable voltageoutput is 1.85 V.

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IN IN

CMI

V Gain V GainAVSS 0.1 V V AVDD 0.1 V

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3.6.2 Analog-to-Digital ConversionFor the TIDA-00463, the ADC needs to have the following requirements:• ≥ 16-bit ENOB• ≥ Three differential inputs• 4.5-V analog supply• 3.3-V digital supply• Operating temperature: –40°C to 125°C

On TI’s portfolio, the ADC that satisfies those requirements is the ADS1248, a precision 24-bit SigmaDelta ADC.

The ADS1248 can have an external reference voltage or an internal one. For the TIDA-00463, the chosenreference voltage for the ADS1248 is the internal one (2.048 V) because it has very low noise (6 μVPP)and a very low drift (10 ppm/°C).

In addition, 2.048 V covers all the signal range (VOUTMAX[V] = 1.85 V) and the LSB, consideringENOB = 16 bits, is equal to 31.25 μV, meaning 32 nT is the minimum measurable magnetic field.Considering the ADS1248 datasheet, to achieve a 16-bit ENOB with AVDD = 4.5 V, AVSS = 0 V,internal reference = 2.048 V, and PGA = 1, the data rate has to be a maximum of 40 SPS. This limits theresponse time of the system because the DRV425 has a much higher bandwidth (30 kHz).

Also keep in mind that the common-mode input (VCMI) must be within the range shown in Equation 4:

(4)

For the TIDA-00463, 1.025 V ≤ VCMI ≤ 3.475 V, where VCMI is around 1.32 V. At the input of eachdifferential channel, an antialiasing filter is present to reduce the differential and common-mode noise.

The ADS1248 sends out the data to the LaunchPad and is configured by this one through SPI.

3.6.3 Board Power SupplyThe TIDA-00463 is powered by the TPS71745, a single-output LDO with a fixed voltage equal to 4.5 V.The input voltage of the TPS71745 is the USB voltage coming directly from the LaunchPad equal to5 V ± 0.25 V. Considering a minimum input voltage equal to 4.75 V, the TPS71745 assures at the output avoltage equal to 4.5 V with which the entire board can be powered through to the 150-mA output currentprovided by it.

One of the other reasons why the TPS71745 is selected is for its temperature range that goes from–40°C to 125°C, which aligns with the other ICs temperature range, making the entire board temperaturerange between –40°C and 125°C.

The current consumption of the TIDA-00463 is around 20 mA; in low power systems, a duty cycle controlof the board power supply could lower the TIDA-00463 current consumption. The enable pin, controlled bythe LaunchPad, on the TPS71745 allows to do that. The TPS71745 also has low noise (30 μVRMS typical)and high bandwidth PSRR.

Do not use a converter because the current flowing on the external inductor generates a magnetic fieldthat could influence the measurements.

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TIDA-00463

DRV425 DRV425

VOUT1

VOUT2REFOUT1

ADS1248

MISO

MOSI

CS

CLK

3.3 V

TPS71745ENABLE

4.5 V

ERROR1OR1 OR2 ERROR2

3.3 V3.3 V

VUSB

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Block Diagram www.ti.com




4 Block Diagram

Figure 8. TIDA-00463 Block Diagram

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DRV425

ADC

FluxgateSensor

and Compensation

Coil

Integrator

Device Control and Diagnostic Reference

DifferentialDriver

Shunt Sense

Amplifier

Fluxgate Sensor Front-End

RSHUNT

AINP AINN

VOUT

REFIN

REFOUT

DRV1DRV2COMP2COMP1

www.ti.com Block Diagram




4.1 Highlighted ProductsFor more information on each of these devices, see their respective product folders at www.ti.com.

4.1.1 DRV425—Fluxgate Magnetic Field SensorFeatures:• High-precision, integrated fluxgate sensor:

– Offset: ±8 µT (Max)– Offset drift: ±5 nT/°C (Typ)– Gain error: 0.04% (Typ)– Gain drift: ±7 ppm/°C (Typ)– Linearity: ±0.1%– Noise: 1.5 nT/√Hz (Typ)

• Sensor range: ±2 mT (Max)– Range and gain adjustable with external resistor

• Selectable bandwidth: 47 or 32 kHz• Precision reference:

– Accuracy: 2% (max); Drift: 50 ppm/°C (max)– Pin-selectable voltage: 2.5 or 1.65 V– Selectable ratiometric mode: VDD / 2

• Diagnostic features: Overrange and error flags• Supply voltage range: 3.0 to 5.5 V

Figure 9. DRV425 Simplified Schematic

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Input

Mux

3rd Order

DS

Modulator

REFP1 REFN1 VREFOUT VREFCOM

REFP0/

GPIO0

REFN0/

GPIO1

Burnout

Detect

Burnout

Detect

DVDD

DGNDIEXC1AVSS

AIN0/IEXC

AIN1/IEXC

AIN2/IEXC/GPIO2

AIN3/IEXC/GPIO3

AIN4/IEXC/GPIO4

AIN5/IEXC/GPIO5

AIN6/IEXC/GPIO6

AIN7/IEXC/GPIO7

ADS1248 Only

SCLK

DIN

DRDY

DOUT/DRDY

CS

START

RESET

AVDD

IEXC2

Internal Oscillator

Voltage

Reference

Serial

Interface

and

Control

VBIAS

GPIO

CLK

ADS1248 Only

ADS1248

PGA

System

Monitor

Adjustable

Digital

Filter

Dual

Current

DACs

VREF Mux

ADS1248 Only

Block Diagram www.ti.com




4.1.2 ADS1248—Precision 24-Bit ADCFeatures:• Data output rates up to 2 kSPS• Single-cycle settling for all data rates• Simultaneous 50- and 60-Hz rejection at 20 SPS• Four differential, seven single-ended inputs (ADS1248)• Low-noise PGA: 48 nVRMS at PGA = 128• Matched current source DACs• Very low drift internal voltage reference: 10 ppm/°C (maximum)• Sensor burn-out detection• Eight general-purpose I/Os• Internal temperature sensor• Power supply and VREF monitoring (ADS1247, ADS1248)• Self and system calibration• SPI-compatible serial interface• Analog supply unipolar (2.7 to 5.25 V) and bipolar (±2.5 V) operation• Digital supply: 2.7 to 5.25 V

Figure 10. ADS1248 Block Diagram

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TPS717xx

GNDEN NR

IN OUTVIN VOUT

1 Fm

Ceramic

0.01 Fm

(Optional)

1 Fm

Ceramic

VEN

www.ti.com Block Diagram




4.1.3 TPS71745—Single Output LDOFeatures:• 150-mA low-dropout regulator with enable• Low IQ: 45 μA (typical)• Available in multiple output versions:

– Fixed output with voltages from 0.9 to 5.0 V using innovative factory EEPROM programming– Adjustable output voltage from 0.9 to 6.2 V

• Ultra-high PSRR:– 70 dB at 1 kHz, 67 dB at 100 kHz, and 45 dB at 1 MHz

• Low noise: 30 μV typical (100 Hz to 100 kHz)• Stable with a 1.0-μF ceramic capacitor• Excellent load/line transient response• 3% Overall accuracy (over load/line/temp)• Overcurrent and over-temperature protection• Very low dropout: 170 mV typical at 150 mA• Small SC70-5, 2×2-mm SON-6, and 1.5×1.5-mm SON-6 packages

Figure 11. Typical Application Circuit for Fixed-Voltage Versions

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Getting Started Hardware www.ti.com




5 Getting Started HardwareTo connect the TIDA-00564 BoosterPack, ensure that the connector index match on the LaunchPad andthe BoosterPack.

Once connected, perform the following actions:1. Connect the micro-USB of the LaunchPad to the PC.2. Ensure in the Windows Device Manager (or alternative naming for other OS) that the board is

recognized.3. Install Energia™ if not yet done.4. Load the sketch on the LaunchPad. For the sketch, contact a TI representative.5. Open the serial monitor to display the read values.

5.1 LaunchPad EcosystemLaunchPads are microcontroller development kits from Texas Instruments focusing on ease of use forevaluation:• Simple USB interface to PC (no more need for an additional debugger to connect to the JTAG port as

the logic is already on the LaunchPad board)• Standardized interface for extension boards (called BoosterPacks)

The LaunchPads comply with the following electrical interface specification, which is available as a pdfhere: http://www.ti.com/ww/en/launchpad/dl/boosterpack-pinout-v2.pdf.

Figure 12. LaunchPad Ecosystem

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http://www.ti.com/ww/en/launchpad/dl/boosterpack-pinout-v2.pdf

200 mil

GND

GND

5 V

GND

GND

3.3 V

40-pin

20-pin

1600 mil

Emulator

MCU

1800 mil

3.3 V

Analog In

UARTRX

TX

GPIO (!)

SPI CLK

GPIO

I2C *SCL

SDA

5 V

GND

Timer Capture

Timer Capture GPIO

GPIO

GPIO

GPIO

GPIO

GPIO

GPIO

GPIO

GPIO

GPIO

PWM Out

PWM Out

PWM Out

PWM Out

LaunchPad

SPI CS Wireless

SPI CS Display

SPI CS Other

GND

GPIO

GPIO

GPIO**

RST

SPIMOSI

MISO

GPIO (!)

GPIO

GPIO

PWM Out

200 milWS

SCLK

SDout

SDin

I2S

1

10

20

11

40

31

21

30

All through-holes on 100-mil grid

( MCU)

( MCU)

Analog In

(!)

Analog In

Analog In

Analog In

Analog In

Analog In

Analog In

Analog Out

Analog Out

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(!)

(!)

(!)

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www.ti.com Getting Started Hardware




5.1.1 BoosterPackBoosterPacks are plug-in modules that fit on top of a LaunchPad. These innovative tools plug into a consistent and standardized connector on theLaunchPad and allow developers to explore different applications enabled by a TI microcontroller.

BoosterPacks are available from Texas Instruments, from third parties, and from the community. They include functions such as capacitive touch,wireless communication, sensor readings, LED lighting control, and more. BoosterPacks are available in 20- and 40-pin variants, and multipleBoosterPacks can plug into a LaunchPad to enhance the functionality of a design.

Figure 13. BoosterPack Pinout

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1 2

Sensors

10 mm

Magnet

10 cm

1 mm

Test Setup www.ti.com




6 Test SetupFor the room temperature test setup, the TIDA-00463 is always fixed in a certain position and the magnetmoves backward and forward towards the board. For the over temperature test setup, both theTIDA-00463 and the magnet are fixed in a certain position.

At room temperature two test setups were implemented: one where the magnet moves in the samedirection of the fluxgate axis, and one where it moves in a perpendicular direction with respect to thefluxgate axis.

6.1 Same Direction Test SetupIn this setup, the TIDA-00463 is fixed and the magnet moves in the same direction of the fluxgate axis(see Figure 14). The magnet in this setup is a 6-mm diameter NdFeB with HC > 1350 kA/m.

The TIDA-00463 is connected to the LaunchPad, and this one is connected to any laptop where on aserial monitor are shown the acquired data. Energia code was implemented to configure the ADS1248and to read back the data, sending them to the serial port.

Figure 14. Same Direction Setup Scheme

Figure 15. Same Direction Test Setup

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12

Sensors

10 mm

Magnet8.5 cm

1 mm

www.ti.com Test Setup




6.2 Perpendicular Direction Test SetupThis setup is similar to the previous one with the only exception that the magnet moves in a perpendiculardirection to the fluxgate axis (see Figure 16).

Figure 16. Perpendicular Direction Setup Scheme

Figure 17. Perpendicular Direction Test Setup

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1 2

Sensors

10 mm

Magnet

5 cm

1 mm

Oven

1 2

Sensors

10 mm

Oven

Test Setup www.ti.com




6.3 Over-Temperature Test SetupTo test over temperature, both the board and the magnet are in a fixed position. The distance of themagnet from the board is equal to 5 cm. Two temperature cycles are performed, one without a magnet inthe oven and one with from –40°C up to 85°C.

Data acquisition happens in the same way of the first two test setups.

Figure 18. Over-Temperature Test Setup Scheme

Figure 19. Over-Temperature Test Setup

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Out

put V

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mV

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0

50

100

150

200

250

300

350

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5605

4687

5

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3605

4687

5

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3605

4687

5

-7.3

3593

7976

8

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www.ti.com Test Data




7 Test DataA first functional test without any magnet and at room temperature is performed to see if the boardbehaves as expected.

1000 measurements were acquired and the results plot in a histogram. The values on the x axis refer tothe differential output voltage of the system in millivolt. Theoretically speaking, this should be zerobecause there is not a magnet around the board, so the only field present should be the common modeone—the earth magnetic field—and it should be cancelled by the differential measurement. However,there are some errors introduced by the fact that this design uses two different shunt resistors for the twodevices, the manufacturer PCB tolerances that affect the alignment of the two fluxgates, and the 10-mmdistance between the two sensors.

The average over 1000 samples of the measured differential output is equal to –7.39 mV (–7.57 µT) whilethe standard deviation equal to 21 μV (22 nT).

The average over 1000 samples of the measured single output voltage for the two DRV425 is equal to–25.43 mV for the first device and –32.84 mV for the second one, corresponding to –26 μT and –34 μT,respectively. Note that this is the value of the earth magnetic field that ranges from ±25 to ±65 μT. Themaximum peak-to-peak noise measured is equal to 300 μV (320 nT). Considering that the chosenbandwidth of the DRV425 is around 30 kHz, the noise density according to the datasheet is 260 nTRMS.However, since the output of the system is the sum of the two DRV425 outputs, the total RMS noise is368 nTRMS (260 nTRMS × √2 ) and the peak-to-peak noise is five to six times the RMS noise (suitable valueis 6.6 times the RMS value) equal to 2.43 μT (2.37 mV).

The measured peak-to-peak noise of the system is at least seven times lower than the estimated one ofthe two sensors, showing the good performances of the TIDA-00463.

Figure 20. Measured Differential Output Voltage (mV)

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Distance (mm)

B (

µT

)

65 70 75 80 85 90 95 10005

10152025303540455055606570

D006

OUT1OUT2Earth Magnetic Field

Distance (mm)

B (

µT

)

20 30 40 50 60 70 80 90 100 110 1200

200

400

600

800

1000

1200

1400

1600

1800

2000

D004

OUT1OUT2

Distance (mm)

B (

µT

)

20 30 40 50 60 70 80 90 100 110 1200

100200300400500600700800900

10001100120013001400

D005

Test Data www.ti.com




7.1 Same Direction Test DataAs shown in Section 6.1, a magnet moves in 1-mm increments from 2 to 10 cm. 2 cm is the closestdistance that the magnet can get to the board before the DRV425 closer to the TIDA-00463 edgesaturates. After 10 cm, the resolution becomes smaller and smaller until 30 cm, after which the magnet isnot detectable anymore. Note that with a bigger and stronger magnet, the measured distance becomesbigger.

In Figure 21, the black curve represents the output in μT of the sensor closer to the board edge, while thered curve represents the output of the other one. Notice how the two curves get closer as the distanceincreases, making the differential measurements of the two smaller. Figure 22 shows the differentialoutput. Note that the resolution become bigger as the distance increases.

Figure 23 is a zoomed version of Figure 21 in the earth magnetic field range. It could be impossible todistinguish between the earth magnetic field and the actual magnetic field generated by the magnet if thedifferential output approach is not adopted (see Section 3.5) or without any calibration.

Figure 21. Measured Magnetic Field of Two DRV425versus Distance

Figure 22. Differential Measured Magnetic Field versusDistance

Figure 23. Measured Magnetic Field of Two DRV425 and Earth Magnetic Field versus Distance

http://www.ti.com


Out

put V

olta

ge (

mV

)

0

25

50

75

100

125

150

175

200

225

250

275

300

-3.5

6176

7816

5

-3.5

4763

6957

1

-3.5

2763

6957

1

-3.5

0763

6957

1

-3.4

8763

6957

1

-3.4

6763

6957

1

-3.4

4763

6957

1

-3.4

2763

6957

1

-3.4

0763

6957

1

-3.3

8763

6957

1

-3.3

6763

6957

1

-3.3

4763

6957

1

-3.3

2763

6957

1

D009

Out

put V

olta

ge (

mV

)

0

20

40

60

80

100

120

140

160

180

-6.1

1596

6796

8

-6.0

9596

6796

8

-6.0

7596

6796

8

-6.0

5596

6796

8

-6.0

3596

6796

8

-6.0

1596

6796

8

-5.9

9596

6796

8

-5.9

7596

6796

8

-5.9

5596

6796

8

-5.9

3596

6796

8

-5.9

1596

6796

8

-5.8

9596

6796

8

-5.8

7596

6796

8

-5.8

6450

2429

9

D010

Distance (mm)

B (

µT

)

30 35 40 45 50 55 60 65 70 75 80 85 90 95-70-60-50-40-30-20-10

010203040506070

D007

OUT1OUT2Earth Magnetic Field

Distance (mm)

B (

µT

)

35 40 45 50 55 60 65 70 75 80 85 90 95-140-130-120-110-100

-90-80-70-60-50-40-30-20-10

0

D008





7.2 Perpendicular Direction Test DataIn this test, the maximum reachable distance with a 1-mm resolution is 85 cm because the generatedmagnetic field hits the fluxgate direction perpendicularly, resulting in weaker results. The lowestmeasurable distance is 3.6 cm because of the board shape (see Figure 17) while the maximum one isaround 25 cm, after which the magnet is not detectable anymore.

Note how the curves describe the magnetic field lines. If the main goal of the system is a large distance,this configuration is not recommended.

Everything else said in Section 7.1 is valid for this test as well.

Figure 24. Measured Magnetic Field of Two DRV425 andEarth Magnetic Field versus Distance

Figure 25. Differential Measured Magnetic Field versusDistance

7.3 Over-Temperature Test Data1000 measurements were acquired for each temperature step (–40°C, 25°C, and 85°C) without anymagnet and with the magnet placed 5 cm from the board. The first test setup measures over temperature.

A histogram is plotted for each temperature step and the standard deviation, mean, and maximum noiseare calculated (see Table 3).

Figure 26. Differential Output (mV) Without MagnetPresent at –40°C

Figure 27. Differential Output (mV) Without MagnetPresent at 25°C

http://www.ti.com


Temperature (°C)

Diff

eren

tial O

utpu

t (m

V)

-45 -30 -15 0 15 30 45 60 75 90-10

-9.2

-8.4

-7.6

-6.8

-6

-5.2

-4.4

-3.6

-2.8

-2

D012

Out

put V

olta

ge (

mV

)0

30

60

90

120

150

180

210

240

-9.5

7617

2828

6

-9.5

5617

2828

6

-9.5

3617

2828

6

-9.5

1617

2828

6

-9.4

9617

2828

6

-9.4

7617

2828

6

-9.4

5617

2828

6

-9.4

3617

2828

6

-9.4

1617

2828

6

-9.4

0869

2359

9

D011





Figure 28. Differential Output (mV) Without Magnet Present at 85°C

Table 3. Over-Temperature Data Without Magnet

TEMPERATURE STANDARD DEVIATION (µV) MEAN (mV) MAX NOISE[PEAK-TO-PEAK] (mV)

–40°C 30 –3.4 0.2325°C 47 –6.0 0.2585°C 30 –9.5 0.17

The maximum peak-to-peak noise measured is 0.25 mV (240 nT), a much lower value than the estimatedone of the two sensors (see Section 7).

The temperature drift calculated as the mean value at 85°C minus the mean value at –40°C divided by theall temperature range is 50 μV/°C (52 nT/°C). This is due by many contributions: the temperature drift ofthe DRV425, TPS71745, and ADS1248, the temperature drift of the internal reference of the ADS1248,and the drift the shunt resistances value over temperature.

Figure 29 shows as the measured differential output change in function of the temperature.

Figure 29. Measured Differential Output versus Temperature Without Magnet

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Out

put V

olta

ge (

mV

)

0

50

100

150

200

250

300

350

68.4

4067

3828

1

68.4

6067

3828

1

68.4

8067

3828

1

68.5

0067

3828

1

68.5

2067

3828

1

68.5

4067

3828

1

68.5

6067

3828

1

68.5

8067

3828

1

68.5

9497

8332

5

D015

Out

put V

olta

ge (

mV

)

0

30

60

90

120

150

180

210

240

79.6

6284

1796

8

79.6

9284

1796

8

79.7

2284

1796

8

79.7

5284

1796

8

79.7

8284

1796

8

79.8

1284

1796

8

79.8

4284

1796

8

79.8

7284

1796

8

79.9

0284

1796

8

79.9

3284

1796

8

79.9

6284

1796

8

79.9

7461

7004

3

D013

Out

put V

olta

ge (

mV

)

0

50

100

150

200

250

300

350

400

74.3

1909

1796

8

74.3

3909

1796

8

74.3

5909

1796

8

74.3

7909

1796

8

74.3

9909

1796

8

74.4

1909

1796

8

74.4

3909

1796

8

74.4

5313

2629

3

D014





The data obtained when the magnet is inside the oven 5 cm from the board are similar to the oneobtained when it is not (see Table 4). The only difference in this case is that the temperature drift is thedouble (97 nT) because in this case it is influenced by the magnet changes over temperature. Theseresults confirm that the system has good performances over temperature as well.

Figure 30. Differential Output (mV) When Magnet at 5 cmFrom DRV425 at –40°C

Figure 31. Differential Output (mV) When Magnet 5 cmFrom DRV425 at 25°C

Figure 32. Differential Output (mV) When Magnet 5 cm From DRV425 at 85°C

http://www.ti.com


Temperature (°C)

Diff

eren

tial O

utpu

t (m

V)

-45 -30 -15 0 15 30 45 60 75 906768697071727374757677787980818283

D016





Table 4. Over-Temperature Data With Magnet 5 cm From the Board

TEMPERATURE STANDARD DEVIATION (µV) MEAN (mV) MAX NOISE[PEAK-TO-PEAK] (mV)

–40°C 50 79.80 0.3125°C 22 74.39 0.1385°C 26 68.50 0.15

Figure 33. Measured Differential Output versus Temperature When Magnet is 5 cm From DRV425

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AINN14

AINP13

COMP116

COMP217

DRV112

DRV211

ERROR19

GND7

GND10

GND18

GND20

BSEL1

OR15

REFIN5

REFOUT4

RSEL03

RSEL12

VDD8

VDD9

VOUT6

PAD21

U2

DRV425RTJR

AINN14

AINP13

COMP116

COMP217

DRV112

DRV211

ERROR19

GND7

GND10

GND18

GND20

BSEL1

OR15

REFIN5

REFOUT4

RSEL03

RSEL12

VDD8

VDD9

VOUT6

PAD21

U1

DRV425RTJR

1

3

5 6

4

2

7

9 10

8

1211

1413

1615

1817

2019

J1

SSW-110-23-F-D

1

3

5 6

4

2

7

9 10

8

1211

1413

1615

1817

2019

J2

SSW-110-23-F-D

+3.3V

SPI_CLK SPI_MISO

GNDGND

SPI_MOSI

SPI_CS

VUSB

10.0kR1 10.0k

R2

10.0kR3 10.0k

R4

OR1ERROR1

ERROR2OR2

+3.3V+3.3V

+3.3V+3.3V

OR1ERROR1

OR2ERROR2

GND

GND

VDD

1µFC21µF

C1

1µFC3

1µFC4

GNDGND

VDD

GNDGND

VDD

OUT1

NR3

EN4

NC5

IN6

2

GND

U3

TPS71745DSER

GND

GND GND

EN

EN

0.01µFC6

GND

VDDVUSB

20.0

R5

20.0

R6

GND

GND

DVDD1

DGND2

CLK3

RESET4

REFP0/GPIO05

REFN0/GPIO16

REFP17

REFN18

VREFOUT9

VREFCOM10

AIN0/IEXC11

AIN1/IEXC12

AIN4/IEXC/GPIO413

AIN5/IEXC/GPIO514

AIN6/IEXC/GPIO615

AIN7/IEXC/GPIO716

AIN2/IEXC/GPIO217

AIN3/IEXC/GPIO318

IEXC219

IEXC120

AVSS21

AVDD22

START23

CS24

DRDY25

DOUT/DRDY26

DIN27

SCLK28

U4

ADS1248IPWR

GND

SPI_CLK

SPI_CSSPI_MISO

SPI_MOSI

GND

0.1µFC8

0.1µFC9

GND

VDD+3.3V

GND

RESETSTART

STARTRESET

47R1347R14

47R1647R15

1000pFC18

470

R7

470

R8

470

R9

470

R10

470

R11

470

R12

GND

1000pFC17GND

1000pFC16

GND

1000pFC15GND

1000pFC13

1000pFC14

GND

GND

1µF

C19

GND

1µFC5

1µFC7

0.01µFC10

0.01µFC11

0.01µFC12

10.0kR17

GND

0R18

0R19

DNP

0R20

0R21

DNP

GND

VDD

GND

0R22

0R23

0R24

DNP0

R25DNP

GND GND

VDD VDD

DRDY47R26

DRDY

0.1µFC22

0.1µFC21

0.1µFC20

0.1µFC23

GNDGND

VDD

GNDGND

VDD

Green1

2D1

470R27

VDD

GND

OUT1

OUT2

OUT1

OUT2

OUT2

VREF

VREF

VREF

OUT1

www.ti.com Design Files




8 Design Files

8.1 SchematicsTo download the schematics, see the design files at TIDA-00463.

Figure 34. Fluxgate-Based Displacement Sensor Schematic

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Design Files www.ti.com




8.2 Bill of MaterialsTo download the bill of materials (BOM), see the design files at TIDA-00463.

8.3 PCB Layout RecommendationsThe board has been designed in a BoosterPack form factor:• Place in a very precise way the two 20-pin connector at each side of the board so that they perfectly

match with the 20-pin connector of the LaunchPad.• For the DRV425 [9]:

– Route current-conducting wires in pairs: route a wire with an incoming supply current next to, or ontop of, its return current path. The opposite magnetic field polarity of these connections cancel eachother. To facilitate this layout approach, the DRV425 positive and negative supply pins are locatednext to each other.

– Route the compensation coil connections close to each other as a pair to reduce coupling effects.– Minimize the length of the compensation coil connections between the DRV1/2 and COMP1/2 pins.– Route currents parallel to the fluxgate sensor sensitivity axis. As a result, magnetic fields are

perpendicular to the fluxgate sensitivity and have limited affect.– Vertical current flow (for example, through vias) generates a field in the fluxgate-sensitive direction.

Minimize the number of vias in the vicinity of the DRV425.– Use nonmagnetic passive components (for example, decoupling capacitors and the shunt resistor)

to prevent magnetizing effects near the DRV425.– Do not use PCB trace finishes with nickel-gold plating because of the potential for magnetization.– Connect all GND pins to a local ground plane.– Do not have a signal plane under the device.

• For the ADS1248 [11]:– Separate analog and digital signals. To start, partition the board into analog and digital sections

where the layout permits. Route digital lines away from analog lines. This prevents digital noisefrom coupling back into analog signals.

– The ground plane can be also split into and analog plane (AGND) and digital plane (DGND), butthis is not necessary. Digital signals can be placed over the digital plane, and analog signals can beconnected over the analog plane. As a final step in the layout, the split between the analog anddigital grounds can be connected to together at the ADC.

– Fill void areas on signal layers with ground fill.– Provide good ground return paths. Signal return currents will flow on the path of least impedance. If

the ground plane is cut or has other traces that block the current from flowing right next to thesignal trace, it will have to find another path to return to the source and complete the circuit. If it isforced into a larger path, that will increase the chances that the signal will radiate and that sensitivesignals will be more susceptible to EMI.

– Use high-frequency bypassing. Do not place vias between bypass capacitors and the active device.Placing the bypass capacitors on the same layer as and as close to the active device will yield thebest results.

– Consider the resistance and inductance of the routing. Often, traces for the inputs have aresistance that reacts with the input bias current and cause an added error voltage. Reducing theloop area enclosed by the source signal and the return current will reduce the inductance in thepath. Reducing the inductance will reduce the EMI pickup and reduce the high frequencyimpedance seen by the device.

– Watch for parasitic thermocouples in the layout, where dissimilar metals are used going from eachinput to the measurement. Differential inputs should be matched for both the inputs going to themeasurement source.

– Keep the reference impedances low.

http://www.ti.com



www.ti.com Design Files




• For the TPS71745:– Place all circuit components on the same side of the circuit board and as near as practical to the

respective LDO pin connections.– Place ground return connections to the input and output capacitor, and to the LDO ground pin as

close to the GND pin as possible, connected by wide, component-side, copper surface area.– Do not use vias and long traces to create LDO component connections, which negatively affects

system performance. This grounding and layout scheme minimizes inductive parasitics, andthereby reduces load-current transients, minimizes noise, and increases circuit stability.

– A ground reference plane is also recommended, which is either embedded in the printed circuitboard (PCB) itself or located on the bottom side of the PCB opposite the components. Thisreference plane serves to assure accuracy of the output voltage, shields the LDO from noise, andfunctions similar to a thermal plane to spread (or sink) heat from the LDO device when connectedto the thermal pad. In most applications, this ground plane is necessary to meet thermalrequirements.

8.3.1 Layout PrintsTo download the layout prints for each board, see the design files at TIDA-00463.

8.4 Altium ProjectTo download the Altium project files, see the design files at TIDA-00463.

8.5 Gerber FilesTo download the Gerber files, see the design files at TIDA-00463.

8.6 Assembly DrawingsTo download the assembly drawings, see the design files at TIDA-00463.

http://www.ti.com






References www.ti.com




9 References

1. Ripka, Pavel, Magnetic Sensor and Magnetometers. Artech House Publishers (2001). ISBN: 978-1580530576.

2. Grüger, Heinrich, Angelika Grüger, and Florian Kaluza, New and future applications of fluxgatesensors. J. Sensors, Sensor and Actuators A, 10.6 (2003): 48–51.

3. Macintyre, Steven A., Magnetic Field Measurement. CRC Press, 1999 (PDF).4. Ripka, Pavel, Advances in Magnetic Fluxgate Sensors, 10.6 (2010): 1108–1116 (PDF).5. Evans, Ken, Fluxgate Magnetometer Explained, (2006; PDF).6. Suitella, Dominggus Yosua and Dr. Ir. Djoko Windarto, MT, High Precision Fluxgate Current Sensor,

(2011; PDF).7. Meeker, D.C., FEMM 4.2, Finite Element Method Magnetics. http://www.femm.info [Accessed on

09/08/2014].8. Larson, Mats G. and Fredrik Bengzon, The Finite Element Method: Theory, Implementation, and

Applications. Texts in Computational Science and Engineering. Berlin, Heidelberg: Springer, (2013).ISBN: 978-3-642-33287-6.

9. Texas Instruments, DRV425 Fluxgate Magnetic-Field Sensor, DRV425 Datasheet (SBOS729).10. Texas Instruments, TPS717xx Low-Noise, High-Bandwidth PSRR, Low-Dropout, 150-mA Linear

Regulator, TPS71745 Datasheet (SBVS068).11. Texas Instruments, 24-Bit Analog-to-Digital Converters for Temperature Sensors, ADS1248 Datasheet

(SBAS426).12. Lenz, James and Alan S. Edelstein, Magnetic sensors and their applications. IEEE Sensors Journal,

6.3 (2006): 631–649 (PDF).

10 About the AuthorGIOVANNI CAMPANELLA is an Industrial Systems Engineer with the Field Transmitter Team in theFactory Automation and Control organization. He earned his bachelor's degree in electronic andtelecommunication engineering at the University of Bologna and his master's degree in electronicengineering at the Polytechnic of Turin in Italy. His design experience covers sensors and analog signalchain (with a focus on fluxgate and analytics sensing technologies) and mixed-signal control of DC-brushed servo drives.

http://www.ti.com


http://engineering.dartmouth.edu/dartmag/docs/macintyre.pdf

https://goo.gl/pgfot7

http://www.invasens.co.uk/FluxgateExplained.PDF

http://core.ac.uk/download/files/379/11731104.pdf

http://www.femm.info

http://www.ti.com/lit/pdf/SBOS729

http://www.ti.com/lit/pdf/SBVS068

http://www.ti.com/lit/pdf/SBAS426

http://goo.gl/5LQ2gH

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fluxgate-based displacement sensor

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